Crystallographic characterization of a novel protein SixA which exhibits phospho-histidine phosphatase activity in the multistep His-Asp phosphorelay.

SixA has been isolated from Escherichia coli as the first protein to exhibit phospho-histidine phosphatase activity. Recent biochemical studies have shown that SixA is involved in the signal transduction of the His-Asp phosphorelay through the dephosphorylation of the histidine-containing phosphotransfer (HPt) domain of the anaerobic sensor kinase ArcB. Crystals of SixA were obtained using a hanging-drop vapour-diffusion method with polyethylene glycol and calcium ions. Preliminary X-ray crystallographic analysis revealed that the crystals belonged to space group P212121 with unit-cell dimensions a = 39.26, b = 48.62 and c = 83.18 A, having one molecule in the crystallographic asymmetric unit. The intensity data were collected up to 1.5 A resolution using synchrotron radiation.